1. Field of the Invention
The present invention relates to scanning optics including optical elements formed of resin and an optical scanning device and an image forming apparatus using the same.
2. Description of the Background Art
A current trend in the art of image forming apparatuses including digital copiers and laser printers is toward the use of scanning optics for high-density image formation. The scanning optics requires a beam spot having a small diameter on a photoconductive element. On the other hand, an optical scanning device includes lenses many of which are formed of resin, which is low cost and implements sophisticated surface configuration.
The problem with resin lenses is that they vary in radius of curvature and refractive index due to varying environment more than glass lenses. The variation of such factors directly translates into the variation of the curvature of field and therefore an increase in beam spot diameter, resulting in defective images. It follows that resin lenses must have the variation of optical characteristics corrected.
Japanese Patent Laid-Open Publication No. 8-292388, for example, discloses a correcting method paying attention to the fact that a positive lens and a negative lens are opposite to each other as to the variation of the curvature of field ascribable to temperature variation. The method taught in the above document uses a resin lens opposite in power to a scanning resin lens and positioned on an optical path between a light source and a deflector, thereby canceling the variation of the curvature of field occurring at the scanning lens due to temperature variation. However, the resin lens intervening between the light source and the deflector has no power in the main scanning direction. The above method therefore cannot correct the variation of the curvature of field to occur at the scanning resin lens in the main scanning direction, failing to prevent the beam spot diameter from increasing in the main scanning direction.
Further, in all embodiments included in the document mentioned above, the resin lens having negative power is implemented as a flat-concave cylindrical lens. Such a lens has a radius of curvature as small as about 5 mm or 8 mm, as described in the embodiments, and is therefore difficult to machine or assemble with accuracy. This is because only the concave surface of the lens has a temperature correcting function.
Japanese Patent No. 2,804,647 teaches a method that corrects the curvature of field in the main scanning direction with a resin lens, which has power identical with, but opposite (negative) to, the power of a scanning resin lens in the main scanning direction. In addition, this method limits the position of the scanning lens for reducing the variation of the curvature of field in the subscanning direction to an acceptable level. Further, the above document proposes a method for correcting the curvature of field in the subscanning direction with a resin cylindrical lens having negative power, i.e., with a temperature correcting function assigned to a single surface. However, the former method critically limits the design freedom of the optics. This is also true with the latter method assigning the correcting function to a single surface.
The resin lens with negative power taught in the above U.S. Pat. No. 2,804,647 has an aspherical surface symmetrical with respect to the axis and a cylindrical surface both of which have negative power. However, the aspherical surface mainly implements the correction of the curvature of field in the main scanning direction and has therefore weak power. By contrast, the cylindrical surface essentially directed toward the correction of the curvature of field in the subscanning direction has strong power. As a result, the cylindrical lens has a small radius of curvature and is therefore difficult to machine or assemble with accuracy. Moreover, to provide a beam spot with desirable shape in the subscanning direction, not only the curvature of field in the geometric optics aspect but also wavefront aberration in the wave optics aspect must be corrected.